In a typical image forming apparatus, an electrostatic image is formed on an image forming member, which may for example be the surface of a rotating drum or of a tensioned moving belt. This invention is concerned with that form of apparatus in which the image forming member is a belt. The belt usually comprises an electrically conductive base having a non-conductive image-carrying surface layer, which is usually a photoconductive surface. The electrostatic image is, for example, formed by charging the photoconductive surface to a first potential, known as the "dark" potential, and then image-wise exposing the charged photoconductor surface to dissipate the charge on image areas.
The electrostatic image is brought into the vicinity of a toner developing device, which is supplied with developer, typically a mixture of a particulate toner and magnetic carrier particles. The electrostatic image is developed by passing the belt in the vicinity of the toner development unit. It is common practice to apply the toner-carrier mixture to the surface carrying the electrostatic charge image by means of a developing unit wherein toner and magnetizable carrier particles are mixed and a layer of the toner-carrier mixture, referred to herein as "developer", is picked up by an applicator such as a rotating sleeve or drum having magnets inside, forming a so-called magnetic brush on a "magnetic roller".
In one type of development unit toner particles are mixed with larger magnetizable carrier particles, to which the toner particles adhere by electrostatic attraction force. The electrostatic charge of the toner and carrier particles is obtained triboelectrically by agitation. The charge sign of the toner particles is opposite to the charge sign of the carrier particles.
On rotating the magnetic roller, the toner particles still adhering to the magnetically attracted carrier particles are brought into a developing zone wherein the toner particles are separated from the carrier particles by the electrostatic attraction forces of the electrostatic latent image to be developed and transfer to the latent electrostatic charge image. The sign of the toner particles, compared with the sign of the charge on the image forming member, determines whether the development is a "direct" or "reversed" development. If the toner and the image forming member have opposite signs, the development is direct; toner particles will be attracted to the charged areas of the image forming member. If the toner and the image forming member have the same sign, the development is "reverse"; toner particles will be attracted to the discharged areas of the image forming member.
A DC developing bias potential of suitable value is applied between the magnetic brush and the base of the belt. The sign of the DC bias potential is the same as that of the base of the belt. The value of the DC bias potential is typically between the value of the potential of the image areas and that of the non-image areas.
Toner particles are attracted to the electrostatic image on the belt to thereby form a toner image. Subsequently the belt, carrying the toner image, comes into contact with a substrate, for example paper in sheet or web form, to which the toner image is transferred. Alternatively, the transfer of the toner image from the belt to the substrate may be by way of one or more intermediate transfer members.
In order to achieve a homogeneous density on the final print, it is necessary that a consistent homogeneous development nip between the image carrying surface and the magnetic roller be established over the total width of the image. While this is readily achieved with drum photoconductors, due to the rigidity of the drum and the magnetic roller, this is more difficult in the case of a belt. Usually the belt is caused to pass over a backing member, such as a backing roller or sliding shoe, in the vicinity of the developing unit. However, the dynamic stability is still less than in the case of a drum photoconductor due to the limited wrapping angle around the backing member and due to the limited mechanical tension in the belt.
When the apparatus is operated in non-ideal conditions, such that the tension, T, on the moving belt (N/mm), the modulus of elasticity, E, of the belt base (N/mm).sup.2, the belt thickness, d (mm), and the angle, .alpha., of contact between the belt and the backing member are such that ##EQU1## a consistent homogeneous development nip between the image carrying surface and the magnetic roller cannot be established and poor results in terms of the uniformity of the print density will be obtained.
In multi-color imaging systems, where a number of developing units are positioned around the path of an electrostatic image carrying belt, it is desirable to run the belt at a high speed, in order to obtain a throughput comparable with mono-chrome systems. Furthermore, in order to reduce the total length of the belt, and the overall size, weight and cost of the apparatus, it is necessary to use developing units in which the magnetic rollers have a relatively low diameter. However, we have found that poor results in terms of image density and image quality can be obtained when the apparatus is operated in non-ideal conditions such that the speed, v.sub.p, of the moving belt (mm), and the diameter, d.sub.MR, of the magnetic roller (mm) are such that ##EQU2##